It is beneficial for the sound system operator to have some visual feedback regarding the operating levels of sound system components. Unfortunately, the monitoring of audio program material is not a trivial task. Audio program material is very complex and constantly changing in an unpredictable manner. A number of methods exist for monitoring a waveform, each with its own pros and cons. Ironically, all of the meters shown in this article will produce different indications when fed the same waveform!

There are at least three significant parameters that can be monitored for audio waveform. These include peak, RMS and average.

Peak

The peak value is simply the maximum instantaneous amplitude of the waveform. It’s value is important because all system components must be able to pass it without deformation. “Clipping” occurs when a component does not have sufficient dynamic range to pass the waveform. The maximum peak output voltage of an audio component is usually specified by measuring the RMS value of the largest single frequency sine wave that the component can pass. This becomes the “full scale” output voltage of the device, and is usually specified in decibels referenced to 1 Volt (dBV) or 0.775 Volts (dBu). Many PC-based wave editors use the actual peak voltage (rather than the RMS value of a sine) as the full-scale reference. Others allow the user to decide, so be sure to check before you start analyzing waves.

The Wave Editor
		A wave editor for a personal computer provides the most complete view of a waveform. Statistics about the wave (peak, RMS, etc.) can be easily determined by calculation. Meters that display the entire dynamic range of the waveform can be constructed in software (left). This type of waveform monitoring is not normally a “real-time” process, limiting its use to the recording studio. Wave editors are useful for testing the properties of real-time meters.

RMS

The RMS value of the waveform describes its energy content. RMS or Root Mean Square is a method of determining the area of the waveform. This value is sometimes referred to as the “effective” value of the waveform, and describes the equivalent DC voltage over some time span that would generate the same power as the complex audio waveform. This “heating value” of a waveform is of particular importance in sound system work because it determines how hot the loudspeaker will get. “True RMS” voltmeters are now commonplace, due primarily to low-cost AID converters and processors. Beware that the time span used for the computation, as well as the display refresh rate may vary from meter to meter.

The Oscilloscope
		The oscilloscope displays amplitude variations as a function of time. While useful for repetitive waves such as sine and square, it is of limited usefulness for viewing real-time audio program material..